An Institute of Electrical and Electronics Engineers (IEEE) 802.16e system employs a cellular scheme basically, and can use the same frequency between adjacent cells because the IEEE 802.16e system supports a frequency reuse coefficient 1. Therefore, a terminal inside the system should discriminate between a sector to which the terminal itself belongs and an adjacent sector among sectors that use the same frequency. For this purpose, each sector conveys a PN code of its own on a preamble, which is a first symbol of each frame transmitted to a terminal, and transmits the same.
A total of 114 preamble PN codes are defined in the IEEE 802.16e system standard, and each code has a code index ranging from 0 to 113. Additionally, the preamble PN code has an ID cell and a segment number. Therefore, a terminal may determine the code index, the ID cell, and the segment number by parsing the preamble PN code. Here, the ID cell has one of 32 values ranging from ‘0’ to ‘31,’ and the segment number has one of 3 values ranging from ‘0’ to ‘2.’ Therefore, all codes cannot have unique combination (of an ID cell and a segment number). Only each code ranging from a code No. ‘0’ to a code No. ‘9’5 of the 114 codes has unique combination (of an ID cell and a segment number). Codes ranging from a code No. ‘96’ to a code No. ‘113’ have the same combinations (of an ID cell and a segment number) as those of the code No. ‘0’ to the code No. ‘95.’
When the preamble PN code is duplicated (same) and used between adjacent sectors, an initial cell search and cell acquisition are impossible. Additionally, sector recognition cannot be made even during a handover. Therefore, adjacent sectors should avoid using the same preamble PN code. Even when duplication of 32 ID cells themselves is simply avoided, duplication of 114 preamble PN codes may occur. Therefore, ID cells should be allocated such that duplication of preamble PN codes between adjacent sectors is minimized in order to prevent a problem in sector discrimination during initial cell search and cell acquisition, and a handover.
In the IEEE 802.16e system, the ID cell is used for various purposes, which has an important influence on the performance of the system. Functions of the ID cell are described below.                The ID cell is used for subchannel permutation in a first downlink Partial Usage of Sub-Channels (PUSC) zone.        The ID cell is used for downlink scrambling together with a segment in the first downlink PUSC zone.        The ID cell is used for uplink scrambling together with a frame number in an uplink.        
The purpose of the conventional ID cell allocation method is to minimize a duplication degree of available ID cells considering only the functions of the ID cell. However, as described above, the ID cell is used as an element forming a preamble PN code, and a duplication degree of the preamble PN code should be also minimized. Therefore, an ID cell allocation method for minimizing ID cell duplication between adjacent sectors, and simultaneously, minimizing duplication of a preamble PN code determined by an ID cell is required.